Alloy composition homogeneity plays an important role in the device performance of III–V heterostructures. In this work, we study the spatial composition uniformity of n-In0.12Ga0.88As/i-In0.2Ga0.8As/p-GaAs core–shell nanopillars monolithically grown on silicon. Cross sections extracted along the axial and radial directions are examined with transmission electron microscopy and energy-dispersive X-ray spectroscopy. Interestingly, indium-deficient segments with width ∼5 nm are observed to develop along the radial 〈112̅0〉 directions in the InGaAs layers. We attribute this spontaneous ordering to capillarity effect and difference in group-III adatom diffusion lengths. The slight fluctuation in indium content (∼4%), however, does not induce any noticeable misfit defects in the pure wurtzite-phased crystal. In contrast, the heterostructure exhibits excellent alloy composition uniformity along the axial [0001] direction. Furthermore, abrupt transitions of gallium and indium are seen at the heterointerfaces. These remarkable properties give rise to extraordinary optical performances. Lasing is achieved in the core–shell nanopillars upon optical pump despite the observed alloy composition fluctuation in the radial directions. The results here reveal the potential of the InGaAs-based core